Optical transmission technologies have been rapidly improving due to the development of optical fiber technologies and light sources such as semiconductor lasers. In particular, wavelength division multiplexing, in which optical signals having different wavelengths are transmitted through a single mode fiber, has been established as the key technology to optical communications. Further, the problem of energy loss in optical signals, which is caused by long distance transmission, has been resolved by the recent development of an Erbium-doped fiber amplifier (“EDFA”).
The wavelength band of 1,530–1,565 nm is most commonly employed in optical transmission technologies. In the case that optical signals in the wavelength band are multiplexed and transmitted through a single optical fiber, each of the optical signals has a different refraction index with respect to each wavelength. The different refractive indices to the optical fiber depending on the wavelength causes the phenomenon of dispersion, in which the optical signals through a single optical fiber over a long distance become spread along the time axis. As the required transmission distance becomes longer, the dispersion effect becomes even more prominent to the degree that the transmitted optical signals overlap each other. Thus, it is difficult to discriminate the optical signals at the receiving end of the optical transmission system.
To compensate for the dispersion of these optical signals, there has been mainly used a tunable dispersion compensator adopting a optical fiber grating. Such dispersion compensator facilitates a connection to an optical cable, provides a low transmission loss, and offers no nonlinear phenomenon of the optical signals. For instance, if a central wavelength of the optical signals is λ1, the optical signals consist of a plurality of wavelengths that exist within the range from λ1−δ nm to λ1+δ nm. In such a case, it is known that the longest wavelength, λ1+δ nm, of the optical signals causes the most severe dispersion along the time axis. This is due to a more slow transmission rate than other wavelengths when its transmission distance becomes longer. On the other hand, the smallest wavelength, λ1−δ nm, of the optical signals causes the lowest dispersion due to a more rapid transmission rate than other wavelengths although its transmission distance becomes longer. Consequently, in order to compensate for the dispersion of the longest wavelength, λ1+δ nm, of the optical signal pulses, it may be desirable to reduce a reflection path in the inner of the optical fiber grating. However, in order to compensate the dispersion of the shortest wavelength, λ1−δ nm, it may be preferable to extend a reflection path within the optical fiber grating, thereby compensating the dispersion of the optical signal pulses caused by long distance transmission.
Methods for controlling dispersion value with the tunable dispersion compensator based on the optical fiber grating may be classified into two methods. According to the first method, (1) the optical fiber grating are divided into several or dozens of parts, (2) the refractive index of the grating is changed by heating and cooling each part at a different temperature in order to adjust the dispersion value. According to the second method, (1) optical fiber grating is attached to a surface of a plate, (2) the plate is bent change the period of the grating, and (3) the dispersion value is adjusted due to the changed period of the grating.
However, in the first method, the variation of refractive indices of the grating parts becomes discontinuous due to the repeated heating and cooling, and there may occur unexpected variations of refractive indices on adjacent parts due to thermal conductions. Thus, the performance of the tunable dispersion compensator is degraded.
In the second method, a bending process is performed. More specifically, one of ends of the metal plate, to which the optical fiber grating is attached, becomes fixed and so that only the other end of the metal plate is moved. Therefore, the period of the optical fiber grating may be varied due to a tensile force and a contractile force induced by the bending. In other words, the period of the optical fiber grating becomes longer when the tensile force is induced, while the period of the optical fiber grating becomes shorter when the contractile force is induced. As such, by varying the period of the optical fiber grating, the dispersion value, which is defined as a variation of group delay time of wavelengths of the optical signals, can be adjusted. However, the second method has shortcomings in that it cannot provide a linear dispersion slope and limits the control range of the dispersion value. This is because only one end of the metal plate is moved in the conventional compensator in order to vary the period of the optical fiber grating.